Clock generator circuits have been developed which produce the commonly-used, 16-times-baud clock frequencies used for UART (universal asynchronous receiver transmitter) operation, but these require quartz crystal oscillators especially prepared to provide the basis for division to obtain the desired clock frequencies.
The standard baud rates commonly used belong to one of six families. Some of the families have only one member. The heads of each family, these being the highest baud within a family, are as follows: 19,200; 7,200; 200; 110; 134.5; and 2,000.
The last three are the only members in the respective families. The first three, when divided by powers-of-two, give rise to other standard family members. One of the difficulties has been that of finding a crystal frequency such that all of the required baud rates, for the particular application, could be derived without appreciable error. Because the required baud-rate sets differ from one generator to another, or because the means of derivation employed differ slightly, each generator may require a different crystal frequency.